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When the Navy ramped-up its participation
in Viet Nam in 1966 and 1967, it found there was a huge demand for small arms
of all kinds. The 7.62mm NATO cartridge had been adopted as Department of Defense
standard in 1957. However, the Navy had large stocks of perfectly good caliber
.30 weapons remaining from World War 2 and Korea.

One way to put a lot of 7.62mm NATO
machine guns in to action was the conversion of the caliber .30 M-1919A4 Browning
machineguns to 7.62mm NATO. The converted guns were designated the Mk 21 Mod
0. These conversions were very common aboard the converted landing craft of
Task Force 117 (Mobile Riverine Force) and its ships [the self-propelled barracks
ships (APB) and non-self propelled barracks ships (APL)].

Ammunition used by the Mk 21 Mod
0 was the same as that used by the M-60 general-purpose machinegun (GMPG). This
ammunition came in a steel can of 200 rounds; four M-80 Ball to one M-62 Tracer
in M-13 links. Getting the linked 7.62mm ammunition to function in the Mk 21
with the M-13 link was an interesting exercise because the M-60 and Mk 21 guns
feed differently.

The M-13 link used by the M-60 is
a strip-out, disintegrating type of link. That is, the loops that hold the cartridge
are open on the bottom. A tab on the second loop snaps into the extracting groove
of the cartridge case to retain the case in the link. Cartridges inserted in
the M-13 links hold the belt together. When a cartridge is stripped from the
link, that link falls off and is discarded. When loaded into the M-60, the two
loops are positioned in the feedway, open side towards the gun's feed tray,
and against the cartridge stops. Forward movement of the bolt strips the cartridge
forward and out of the link to fall in front of the bolt. The bolt continues
forward to chamber, lock, and fire the cartridge in the barrel.

The Browning machineguns all use
pull-out type disintegrating links. Browning caliber .30 links have solid loops.
Cartridges inserted in the links hold the belt together. Friction holds the
cartridges in the links. The Browning has a different feeding cycle than the
M-60. When the gun is cocked, the Browning's extractor pulls the cartridge from
the link and feeds into the T-slot of the bolt as the bolt moves to the rear.
The cartridge is chambered on the forward stroke of the bolt and the next round
is picked-up for feeding. This is exactly opposite of the M-60.

In order to make the M-1919A4 in
caliber .30 into the Mk 21 Mod 0 in 7.62mm NATO, Naval Ordnance Station Louisville
had to:
· Install a 6-inch flash suppressor on the end of the muzzle bearing
at the end of the barrel jacket.
· Replace the .30 barrel with a 7.62mm barrel.
· Modify the bolt and feed cover to feed the 7.62mm ammunition.
· Add a second belt holding pawl to position the belted 7.62mm ammunition.
· Add a spacer block in the feedway to help guide the shorter 7.62mm
ammunition and act as a round stop to prevent the use of caliber .30 ammunition.
· Stamp the receiver sideplate with the markings: "Machine Gun,
7.62mm, Mk 21 Mod 0."

When the Mk 21 Mod 0 was loaded with
the belted 7.62mm ammunition, the cover was opened, the open side of the links
were placed on the feedway face-up (that is, towards the operator), the extractor
was snapped over the first cartridge in the belt, and the cover closed. This
was done with the bolt in the forward position. When the cocking handle was
pulled to the rear, the extractor pulled the first round out of the M-13 link
and positioned it in the T-slot of the bolt. The extractor switch cam, riveted
to the left wall of the receiver, pushed the extractor down to move the cartridge
into firing position. The bolt then went forward to chamber the round into the
barrel. At the same time, the extractor switch cam lifted the extractor off
the chambered cartridge to pickup the incoming second round as the bolt went
into battery (that is, all the way forward and locked). The gunner then pulled
the trigger at the back of the receiver to fire the gun.

The Mk 21 Mod 0 machine gun was mounted
in a cradle that could accommodate it or the caliber .50 M-2HB machinegun. The
cradle mounted an ammunition tray what held either 200 rounds of 7.62mm linked
ammunition or 100 rounds of caliber .50 linked ammunition. For Navy use, these
cradles were equipped with armored shields to protect the gunner and the ammunition.

Photo: US
Navy SEAL Team ONEThe HSSC belonging to the
MST-3 detachment at Nha Be looking aft from the bow ramp, in late 1967 or early
1968. Note the lack of in-country modifications, such as bar armor to defeat
shaped-charge rocket projectiles. The breech of the Mk 2 Mod 0 81mm mortar is
seen to the left of the sailor with his back to the photographer. (The gunner
probably kept the barrel depressed to make sure any rainwater would not accumulate
in the bottom and cause the mortar rounds to misfire.) Two Mk 21 Mod 0 machineguns
are on either side of him. The barrels of two .50 machineguns are behind them.